1. Field of the Invention
The present invention relates to a solid oxide fuel cell applicable to a power generating apparatus and, more particularly, to a solid oxide fuel cell, having a simple structure that does not require hermetic sealing, in which a cathode electrode layer and an anode electrode layer, each having a current collecting electrode, are formed on a solid oxide substrate, wherein power can be generated by directly exposing the fuel cell to a premixed gas combustion flame produced by burning, the thermal shock due to rapid heating by a flame is alleviated, and cracking at the periphery of the substrate is prevented from occurring.
2. Description of the Related Art
Fuel cells so far developed can be classified into various types according to the method of power generation, one being the type of solid oxide fuel cell that uses a solid electrolyte. In one example of the fuel cell that uses a solid electrolyte, a calcined structure made of yttria (Y2O3)-doped stabilized zirconia is used as an oxygen-ion-conducting solid oxide substrate. This fuel cell comprises a cathode electrode layer formed on one surface of the solid oxide substrate and an anode electrode layer on the opposite surface thereof, and oxygen or an oxygen-containing gas is supplied to the cathode electrode layer, while a fuel gas such as methane is supplied to the anode electrode layer.
In this fuel cell, the oxygen (O2) supplied to the cathode electrode layer is converted into oxygen ions (O2−) at the boundary between the cathode electrode layer and the solid oxide substrate, and the oxygen ions are conducted through the solid oxide substrate into the anode electrode layer where the ions react with the fuel gas, for example, a methane gas (CH4), supplied to the anode electrode layer, producing water (H2O), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO). In this reaction process, the oxygen ions release electrons, and a potential difference therefore occurs between the cathode electrode layer and the anode electrode layer. When lead wires are attached to the cathode electrode layer and the anode electrode layer, the electrons in the anode electrode layer can flow into the cathode electrode layer via the lead wires, and the fuel cell thus generates electric power. The operating temperature of this fuel cell is about 1,000° C.
However, such a type of solid oxide fuel cell requires the provision of separate chambers, one being an oxygen or oxygen-containing gas supply chamber on the cathode electrode layer side and the other a fuel gas supply chamber on the anode electrode layer side. Then, the fuel cell is heated to the driving temperature by a heater provided outside the separate chamber. However, if heated rapidly, cracking occurs in the solid oxide fuel cell and the generation of power is no longer possible, therefore, it has been necessary to gradually heat the fuel cell, when it is started, over a long period of time. Furthermore, as the fuel cell is exposed to oxidizing and reducing atmospheres at high temperatures, it has been difficult to increase the durability of the solid oxide fuel cell.
On the other hand, there has been developed a fuel cell of the type that comprises a fuel cell fabricated by forming a cathode electrode layer and an anode electrode layer on opposite surfaces of a solid oxide substrate, and that generates an electromotive force between the cathode electrode layer and the anode electrode layer by placing the fuel cell unit in a fuel gas mixture consisting of a fuel gas, for example, a methane gas, and oxygen. The principle of generating an electromotive force between the cathode electrode layer and the anode electrode layer is the same for this type of fuel cell as for the above-described separate-chamber type fuel cell but, as the whole fuel cell can be exposed to substantially the same atmosphere, the fuel cell can be constructed as a single-chamber type cell to which the fuel gas mixture is supplied, and this serves to increase the durability of the fuel cell.
However, in this single-chamber fuel cell also, as the fuel cell has to be operated at a high temperature of about 1,000° C., there is the danger that the fuel gas mixture may explode. Here, if the oxygen concentration is reduced to a level lower than the ignitability limit to avoid such danger, there occurs the problem that carbonization of the fuel, such as methane, progresses and the fuel cell performance degrades. In view of this, there has been developed a single-chamber fuel cell that can use a fuel gas mixture whose oxygen concentration is adjusted so as to be able to prevent the progress of carbonization of the fuel while, at the same time, preventing an explosion of the fuel gas mixture.
The fuel cell so far described is of the type that is constructed by housing the fuel cell in a chamber having a hermetically sealed structure, on the other hand, and there is proposed an apparatus that generates power by placing a solid oxide fuel cell in or near a flame and thereby holding the solid oxide fuel cell at its operating temperature by the heat of the flame.
The fuel cell used in the above-proposed power generating apparatus comprises a zirconia solid oxide substrate formed in a tubular structure, a cathode electrode layer as an air electrode formed on the inner circumference of the tubular structure, and an anode electrode layer as a fuel electrode formed on the outer circumference of the tubular structure. This solid oxide fuel cell using the solid electrolyte is placed with the anode electrode layer exposed to a reducing flame portion of a flame generated from a combustion device to which the fuel gas is supplied. In this arrangement, radicals, etc. present in the reducing flame can be utilized as the fuel, while air, as an oxygen-containing gas, is supplied by convection or diffusion to the cathode electrode layer inside the tubular structure, and the solid oxide fuel cell thus generates electric power.
The above-described single-chamber fuel-cell obviates the necessity of strictly separating the fuel and the air as was the case with conventional solid oxide fuel cells, but instead has to employ an air-sealing construction. Further, to increase the electromotive force, a plurality of flat plate solid oxide fuel cells are stacked one on top of another and connected together using an interconnect material having high heat resistance and high electrical conductivity so as to be able to operate at high temperatures. As a result, the single-chamber fuel-cell device constructed from a stack of flat plate solid oxide fuel cells has the problem that the construction is not only large but also costly.
Furthermore, in operation, the temperature is gradually raised to the high operating temperature in order to prevent cracking of the solid electrolyte fuel cells, therefore, the single-chamber fuel-cell device requires a significant startup time, thus causing extra trouble to operate.
In contrast, the above-proposed solid oxide fuel cell of tubular structure employs a construction that directly utilizes a flame and this type of fuel cell has the characteristic of being an open type, the solid electrolyte fuel cell not needing to be housed in a hermetically sealed container. As a result, this type of fuel cell can reduce the startup time, is simple in structure, and is therefore advantageous when it comes to reducing the size, weight, and cost of the fuel cell. Further, as a flame is used directly, this type of fuel cell can be incorporated in a conventional combustion apparatus, or an incinerator or the like, and is thus expected to be used as a power supply apparatus.
However, in this type of fuel cell, as the anode electrode layer is formed on the outer circumference of the tubular solid oxide substrate, radicals due to the flame are not supplied, in particular, to the lower half of the anode electrode layer, and effective use cannot be made of the entire surface of the anode electrode layer formed on the outer circumference of the tubular solid oxide substrate. This has degraded the power generation efficiency. There has also been the problem that, as the solid oxide fuel cell is directly and unevenly heated by the flame, cracking tends to occur due to rapid changes in temperature.
In view of the above situation, Japanese Unexamined Patent Publication (Kokai) No. 2004-139936, for example, proposes a power generating apparatus using a solid oxide fuel cell as a handy power supply means, wherein improvements in durability and power generation efficiency and reductions in size and cost are achieved by employing a solid oxide fuel cell of the type that directly utilizes a flame produced by burning a fuel, and by making provisions to apply the flame over the entire surface of the anode electrode layer formed on a flat plate solid oxide substrate.
The solid oxide fuel cell used in the above-proposed power generating apparatus has a flat plate solid oxide substrate circular or rectangular in shape, a cathode electrode layer 2 as an air electrode formed on one surface of the substrate, and an anode electrode layer as a fuel electrode formed on the surface opposite to the one surface. The cathode electrode layer and the anode electrode layer are constructed so as to be arranged in opposition to each other via the solid oxide substrate.
As the solid oxide fuel cell is formed in a flat plate shape, the flame produced by a combustion device can be applied uniformly over the anode electrode layer of the solid oxide fuel cell, that is, compared with the tubular type, the flame can be applied more evenly. Furthermore, with the anode electrode layer disposed facing the flame, hydrocarbons, hydrogen, radicals (OH, CH, C2, O2H, CH3), etc. present in the flame can be easily utilized as the fuel to generate power based on the oxidation-reduction reaction. Further, the cathode electrode layer is exposed to an oxygen-containing gas, for example, air and, therefore, the cathode electrode layer can be maintained in an oxygen-rich condition.
By the way, for the solid oxide fuel cell used in the power generating apparatus disclosed in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 2004-139936, it has been proposed that the metal meshes or the metal wires be embedded or fixed at least in or to one of the anode electrode layer and the cathode electrode layer of the solid oxide fuel cell in order to prevent the power generating performance of the solid oxide fuel cell from degrading even if cracking occurs due to it being exposed directly to the flame.
As described above, one solid oxide fuel cell is formed on one solid oxide substrate, however, there may be the case where a plurality of fuel cells are formed on one solid oxide substrate as the solid oxide fuel cell and further the case where the solid oxide fuel cell is formed on each of a plurality of solid oxide substrates having a small area and the individual fuel cells are electrically connected to each other via wires to form a single fuel cell. In these cases also, it has been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2005-71628, Japanese Unexamined Patent Publication (Kokai) No. 2005-63692, etc., that the metal meshes or the metal wires be embedded or fixed at least in or to one of the anode electrode layer and the cathode electrode layer of each fuel cell in order to prevent the power generating performance of the solid oxide fuel cell from degrading even if cracking occurs.
As described above, in the solid oxide fuel cell used in the so far proposed power generating apparatus, the metal meshes or the metal wires are embedded or fixed at least in or to one of the anode electrode layer and the cathode electrode layer of the solid oxide fuel cell in order to prevent the power generating performance of the solid oxide fuel cell from degrading even if cracking occurs due to being exposed directly to the flame. This means that because of the presence of the metal meshes or the metal wires, the individual fuel cells cracked and disintegrated in pieces are electrically connected in parallel as a result, therefore, the generation of power is not affected over the whole of the solid oxide fuel cell.
However, even in the case where the metal meshes or the metal wires are provided in each electrode layer, the individual fuel cells cracked and disintegrated in pieces are electrically connected in parallel by the metal meshes or the metal wires and the ability to generate electric power is maintained but, as described above, at the periphery part of the solid oxide substrate 1, the substrate surface is exposed with a predetermined width, therefore, there has been the problem that cracking occurs also in the periphery part of the solid oxide substrate on which none of the electrode layers is formed and, as a result, the solid oxide fuel cell cannot be used any more.
It is accordingly an object of the present invention to provide a solid oxide fuel cell having a simple structure, that does not require hermetic sealing, in which a cathode electrode layer and an anode electrode layer are formed on a solid oxide substrate, wherein electric power can be generated by directly exposing the fuel cell to a premixed gas combustion flame produced by burning, measures are taken to prevent cracking in the periphery part of the solid oxide substrate, the thermal shock due to rapid heating by the flame is alleviated, and cracking of the entire substrate is prevented from occurring.